The present invention relates to an image forming apparatus for use electrophotography, and particularly relates to speeding up the copying operation with regard to the electrostatic charge characteristic of the photoreceptor.
Image forming apparatuses for use with electrophotography are equipped with drum-like Se or OPC photoreceptors for image formation. The electric potential characteristic of ordinary photoreceptors will be described below.
FIG. 13 is a graph illustrating the electric potential characteristic of an OPC photoreceptor during the continuous operation, i.e., the making of more than one copy sheet.
OPC photoreceptors especially tend to have their electric potentials during first image forming cycle (the dotted line in FIG. 13) less than that of the reference electric potential (the chain line in FIG. 13). In this figure the reference electric potential Vs is a surface electric potential for forming a latent image in such a way that a print and a ground are separated by 0.2 print clarity.
To obtain a toner image providing a proper print clarity, the surface of the photoreceptor, which is being rotated, is charged and then discharged during preprocessing so that the photoreceptor can be conditioned to have the electric potential during first image forming cycle as high as the reference electric potential Vs. Then, the revolving photoreceptor goes through the image forming cycle in which charging, optical image formation, toner depositing, separation and toner fixing are carried out one after another in order. Finally, the surface of the photoreceptor is cleared of dust, residual toner and a residual electric potential during postprocessing.
However, the above-mentioned requirement for preprocessing, image forming and postprocessing creates an obstacle to speeding up the copying operation, when more than one copy sheet is produced continually e.g., when a book is being copied.
Therefore, we have attempted to improve the image forming processing with attention to the electric potential characteristic of the photoreceptor.
FIG. 11 shows relation between a period of non-operational time for a photoreceptor drum (represented by on the abscissa) and surface electrical potential (Vf) which is generated on the electrically charged photoreceptor drum when it is exposed to light reflected on a document whose reflection density (OD) is 0.2 . This is regarded as sensitivity corresponding to the non-operational interval of the photoreceptor drum.
For example, when a photoreceptor drum, which has been electrically charged to a condition of 700 V, is exposed with the above-described reflected light, the surface electrical potential generated on the surface of the drum becomes 300 V. Further when development bias voltage of 180 V is applied to the drum, an actual visible image is formed due to the resetting development voltage of 120 V, which is the difference between the surface electrical potential and the development bias voltage.
During the above-mentioned process, the surface electrical potential sometimes does not reach 300 V, depending on fatigue of the drum, even when the drum is exposed with the aforesaid reflected light. Specifically, a photoreceptor drum kept in a non-operational status for a period of 20 minutes has high sensitivity as shown in the FIG. 11. When it is electrically charged under the condition of 700 V and is exposed to the reflected light, its surface electrical potential (Vf) goes down to 270 V. However, this reduction takes place only in the first cycle after the non-operational time period, and the surface electrical potential of about 300 V occurs in the second cycle and thereafter (i.e., non-operation interval is zero).
Namely, as apparent from the FIG. 11, the surface electrical potential Vf on a photoreceptor, generated when it is exposed to the reflected light from a document, decreases gradually as a function of the length of time elapsed after the ejection of an image-transfer sheet, and the surface electrical potential is lower than Vd herein referred to as the boundary electric potential Vd after an elapsed time of about 10 minutes, which has as been discovered by the inventors of this invention. More specifically, we have discovered that, with the surface electric potential characteristic of the photoreceptor, it is possible to omit preprocessing without creating a difference of in the surface electric potential between the first copying operation and the second or later copying operation, and without impairing the proper print clarity as long as the surface electric potential Vf is higher than the boundary electric potential Vd. Even when more than one copy sheet is produced continually, sufficiently clear copy sheets can be produced by omitting preprocessing, executing the image forming and postprocessing, and then repeating the same operation. The boundary electric potential Vd is the surface electric potential of the photoreceptor by which it is decided whether preprocessing is necessary or unnecessary.
Therefore, an object of the present invention is to provide an image forming apparatus for producing copy sheets with proper print clarity while speeding up the copying operation.
Next, the platen glass in conventional image forming apparatuses can rise above a certain temperature level, harming the user physically and psychologically. It is therefore necessary to prevent this temperature rise by setting a lower CPM value (the CPM value is the number of copy sheets produced per minute) when the platen glass is expected to be hotter than a certain temperature.
For instance, when 600 copy sheets are produced by a conventional image forming apparatus with a room temperature 25.degree. C. and an initial platen glass temperature 25.degree. C., the CPM value of 20 copy sheets per minute is lowered to that of 15 copy sheets per minute when the platen glass reaches 68.degree. C. (e.g., 500 copy sheets produced continuously). This lowering of the CPM value prevents a further temperature rise even if the remaining 100 copy sheets are produced immediately after the initial 500 an external. Then, clock or an internal clock in the CPU of the apparatus measures the time before the start of the next copying operation. If 600 copy sheets are produced again, the CPM value is lowered (more than one CPM value can be set) at the instant 400 copy sheets are produced, because of the residual heat.
Namely, while powdered, conventional image forming apparatuses estimate the temperature of the platen glass from the number of copy sheets which are produced from how long the copy operation is stopped, and from the room temperature. An appropriate CPM value is then selected. The apparatus can use either an independent clock or an internal clock in the CPU to measure time. Many conventional apparatuses have the room temperature set to 30.degree. C., which is their specified maximum value.
In conventional image forming apparatuses described above, however, when the power is turned off, the clock or CPU stops, and therefore it is impossible to measure elapsed time after the copying operation is stopped (hereafter referred to as an "off interval"). Since the apparatus is totally reset once the power is turned off, starting the apparatus again by turning on the power immediately after the reset is likely to cause overheating, because of the underestimation of the temperature of the platen glass.
When a small number of copy sheets are produced intermittently by turning on and off the power in order to estimate the platen glass temperature it is necessary to obtain the value T.sub.0 given by the following equation: EQU T.sub.0 =(T.sub.1 +T.sub.2 +T.sub.3)
where T.sub.0 results from adding first operation time T.sub.1, an off interval T.sub.2 and the second operation time T.sub.3 occurring after the power is turned on again. Here, even if conventional apparatuses are capable of storing the operation time T.sub.1 in memory, the failure to estimate an off interval T.sub.2 make it impossible to obtain T.sub.0.
An idea for coping with this problem has been to use what is called a backup power source and to measure the off interval by using the backup source when the power is turned off. This approach has not been feasible because it makes the whole apparatus more complicated and costly.
In order to prevent overheating there has been a simpler idea of mounting a temperature sensor near the platen. However, this is difficult to do it without blocking the light reflected from a document to be copied. Overcoming this difficulty has proved too expensive.
Therefore, a second object of the present invention is to provide an image forming apparatus capable of measuring the temperature of the platen glass without using a clock or a temperature sensor which were added specially for such measurement.